The present invention relates to an ion concentration measuring apparatus comprising a semiconductor ion sensor.
A semiconductor ion sensor utilizing a field effect in a semiconductor body has been developed and is called Ion Sensitive Field Effect Transistor (ISFET). Such an ISFET has been disclosed in Japanese Patent Application Laid-open Publications Nos. 139,289/76 and 26,292/77. In ISFET, on a gate of an insulated gate field effect transistor manufactured by a well developed IC technique, is formed a chemical selective film containing ion-exchange substances or enzymes, and a potential at an interface between the chemical selective film and an electrolyte is detected to measure specific ion concentration and substances acting upon the enzymes in the electrolyte.
The ion concentration measuring apparatus comprising the above mentioned ISFET has been disclosed in, for instance, Japanese Patent Application Laid-open Publication No. 136,396/79.
FIG. 1 is a schematic view showing the above mentioned known ion concentration measuring apparatus, in which a bias voltage applied to a reference electrode is so controlled that a given constant current flows between the source and drain of an ISFET sensitive to specific ion in a test liquid, and then a concentration of the specific ion and an ion activity are detected in accordance with said bias voltage value. In FIG. 1, a container 1 contains a test liquid 2 and an ISFET 3 is immersed in the test liquid 2 together with a reference electrode 5 in such a manner that at least a gate portion 4 of ISFET 3 is made in contact with the test liquid 2. Between a drain electrode 6 of ISFET 3 and the earth is connected a constant voltage supply source 7, and a source electrode 8 and a semiconductor substrate 9 of ISFET 3 are connected via a reference resistor 10 to the earth. The reference electrode 5 is connected to a potential control circuit 11 including a bias voltage source, by means of which the bias voltage is so controlled that the potential of the source electrode 8 is set to a predetermined value and thus a predetermined current flows between the source and drain. Then the voltage value across outputs 12 and 13 is measured with the aid of a voltmeter such as a valve voltmeter.
In principle, the drain current of the FET is changed in accordance with two factors, i.e. the voltage across the drain and source and the voltage across the gate and source. Further, if the voltage across the drain and source is always maintained above a certain level, the drain current is substantially proportional to the voltage across the gate and source. Such an operating region is called a saturation region. In the ion concentration measuring apparatus shown in FIG. 1, the ISFET 3 is operated in the saturation region and thus the drain current is determined by the voltage across the gate and source.
In FIG. 1, the gate potential V.sub.G of ISFET 3 is equal to a sum of a potential E.sub.R of the test liquid 2 set by the reference electrode 5 and an interface potential E.sub.G generated at the chemically sensitive film of the gate portion 4 in response to the ion activity of specific ion in the test solution 2 (V.sub.G =E.sub.R +E.sub.G). Further, if it is assumed that a resistance value of the resistor 10 is R, the voltage of the constant voltage supply source is E.sub.B and the drain current is I.sub.D, then the voltage V.sub.DS across the drain and source may be expressed by V.sub.DS =E.sub.B -I.sub.D R. Then, a deviation dI.sub.D of the drain current I.sub.D may be represented as follows. ##EQU1## As explained above, when the ISFET 3 is driven in the saturation region, the change in the voltage V.sub.DS across the drain and source does not have any influence upon the drain current I.sub.D. Therefore, in the above equation, it becomes ##EQU2## and ##EQU3## is obtained. That is to say, when E.sub.R +E.sub.G is constant, the drain current I.sub.D is kept also constant.
In the saturation region, when the ion activity of the test liquid 3 varies, the interface potential E.sub.G at the chemically selective film of the gate portion 4 is changed into E.sub.G +.DELTA.E.sub.G, and thus the drain current I.sub.D is varied. Therefore, the voltate I.sub.D R across the reference resistor 10 is also changed. The potential control circuit 11 operates to adjust the potential E.sub.R of the test liquid 2 via the reference electrode 5 in such a manner that the drain current I.sub.D remains at the given value. In this case, the variation .DELTA.E.sub.R of the potential of the test liquid 2 serves to change from dI.sub.D =0 to .DELTA.(E.sub.R +E.sub.G)=0, and therefore, .DELTA.E.sub.R =-.DELTA.E.sub.G. By measuring the variation .DELTA.E.sub.R of the potential of the test liquid 2, i.e. the variation of the potential at the reference electrode 5 across the outputs 12 and 13, it is possible to detect -.DELTA.E.sub.G, so that the variation in the ion activity can be derived therefrom.
In the ion concentration measuring apparatus shown in FIG. 1, since the ISFET 3 is driven by the constant voltage, an electrical overload condition does not occur and thus the operation is stable. However, since the variation in the ion activity is detected by changing the potential E.sub.R of the test liquid 2, it is absolutely necessary to electrically isolate the container 1 in a positive manner and it is principally impossible to simultaneously effect measurements with the aid of a plurality of ISFETs.